A Modular Multilevel power Converter (MMC), also known as Chain-Link Converter (CLC), comprises a plurality of converter cells, or converter (sub-) modules, serially connected in converter branches that in turn may be arranged in a wye/star, delta, and/or indirect converter topology. Each converter cell comprises, in the form of a half-bridge or full-bridge circuit, a capacitor for storing energy and power semiconductor switches such as insulated gate bipolar transistor (IGBT) devices, gate-turn-off thyristor (GTO) devices, or MOSFETs for connecting the capacitor to the converter branch with one or two polarities. The voltage per converter cell capacitor may be between 1 kV and 3 kV; whereas the voltage of a converter branch may be in a range from 10 kV to several 100 kV. An MMC controller with a processor and corresponding software, or an FPGA, is responsible for controlling the converter cells and operating the power semiconductor switches based on a pulse width modulation scheme.
MMCs may be used in electric power transmission systems as ac-only Static VAR Compensators (Statcoms) and/or Flexible AC Transmission Systems (FACTS) devices for static power-factor correction as well as for voltage quality and stability purposes. A Statcom provides reactive power support to an electric power transmission network or grid to which the Statcom is connected by producing or absorbing reactive power.
An operating MMC has a certain amount of stored energy which must be provided to the converter before connecting the converter to the electric grid. To that purpose, charging of the converter cell capacitors is conventionally executed by way of passive charging or by way of active charging under control of dedicated charging controllers.
Connection of the MMCs to the electric grid generally involves a main transformer, whereby for Statcom applications the main transformer may be integrated with the MMC converter and/or arranged inside the same enclosure as the MMC. Transformer inrush is an undesired or often unacceptable effect of directly connecting a transformer to an electric grid. Uncontrolled, and specifically unlimited, transformer inrush currents may lead to saturation of the transformer core and corresponding inductance drop. Therefore, it may be required to first “pre-magnetize” the transformer such that upon grid connection there is no or little inrush current. Transformer pre-magnetization is also known as “flux alignment” or “transformer synchronization” implying that the primary, or grid-side voltage of the transformer is aligned, or synchronized, to both amplitude and phase of the AC grid voltage.
According to the patent application EP 1024574 A2 a transformer connected to an AC network via a power switch in an unsynchronized manner causes both inrush currents that are detrimental to the switch, as well as residual DC currents through unbalanced transformer core saturation that are inacceptable for traction applications. Therefore, EP 1024574 A2 discloses performing a regulated or controlled, network synchronous transformer magnetization by means of a pulse current converter connected to a secondary winding of the transformer, and using system parameters such as network voltage, network current, and input current of the pulse current converter. The transformer magnetization takes place during a few AC cycles prior to connection of the transformer to an AC voltage network, by means of a pre-charging DC/DC converter device connected to an intermediate DC circuit of the pulse current converter. The pulse current converter operates synchronous to the network voltage, and the magnetizing current generated by the converter is measured and the modulation is adapted accordingly to avoid DC current components. Transformer core saturation effects are inferred from the magnetizing current exceeding a threshold, or from an increase in a time derivative of the magnetizing current.
In this context, ramping of a transformer magnetizing voltage by means of an inverter results in small saturation currents that align the transformer flux and that are lower than an inverter current rating. Hence no dedicated intervention or corresponding controller is needed, which is particularly important for high power inverters, including IGCT based converters, which have a smaller control bandwidth compared to MMC converters. The voltage ramping may take tens or hundreds of cycles depending on the transformer characteristics and an external power source such as the DC/DC converter of EP 1024574 may thus be indispensable.
The patent application EP 1858147 A2 discloses a current conversion circuit with a power transformer connected to an alternating current supplying input via switching units for receiving an alternating current, and with a rectifier connected to output of the power transformer to provide a direct current to a capacitor connected between output terminals of the rectifier. A capacitor pre-charging circuit with a three phase auxiliary transformer connected to the output of the power transformer is used for pre-charging the capacitor and concurrently ensures magnetization of the power transformer before switching of the power transformer, and hence preventing high switching currents in the input of the transformer due to saturation of the transformer. The appropriately sized auxiliary transformer with a power rating sufficient to pre-magnetize the power transformer as well as to charge the converter incurs additional cost.